1. Field of the Invention
The present invention relates to differential digital modulation and demodulation systems.
2. Description of the Prior Art
In differential modulation systems such as delta modulation systems and differential pulse code modulation systems a digital word is generated at the transmitting side of the system at the digital modulator as a function of the difference between a sampled value of an input analog signal and a tracking value which changes in quantized steps from sampling time to sampling time which is fundamentally formed by means of a summation of all preceding differential values. An integrator in a differential decoder is discharged or charged in a feedback path of the digital modulator as well as at the receiving side in a digital demodulator, depending upon the measure of the digital signal words. In modulation and demodulation systems operating as described above there is a problem: In the usual case of equidistant sampling times small differences between the sampled value and the tracking value are derived given relatively small sampling periods. That is, small encoding errors are derived even if the input analog signal exhibits a relatively steep curve. The high modulation clock rate, however, requires a correspondingly large transmission channel capacity, for message transmission, or a correspondingly large memory capacity, for message storage. If the sampling period is enlarged, the modulation clock rate is correspondingly lower, however, the tracking values cannot then follow a steep curve for the analog input signal with sufficient precision, and larger encoding errors are thus produced.
In an attempt to achieve an improved exploitation of the transmission channel capacity, a so-called asynchronous delta modulation system has been proposed in an article appearing in Electronics and Communications in Japan, Vol. 49, 1966 at pages 34-42. In the system disclosed in that article, a new digital signal word (a delta modulation bit) is formed in proper phase at such a point in time at which the analog signal has changed by a predetermined amount. Such an asynchronous modulation system, however, also requires an in-phase signal loading on the demodulator side, that is, a signal loading which retains the chronological distance of the delta modulation bits. This requirement excludes the insertion of the digital signal elements into a rigid grid which may deviate from the time frame of the asynchronous modulation system such as, for example, the pulse frame of a time-division multiplex transmission system for message transmission or the storage in a memory for message storage.